1. Field of the Invention
This invention relates to a shield driving process for enlargement wherein the periphery of an existing tunnel already constructed and lined with a primary shield segment assembly is enlarged by a circumferentially driving shield machine.
2. Description of Background Art
A process for enlarging a portion of an existing tunnel has been hithertofore known wherein a shaft is extended from the surface of the earth so as to reach a predetermined region to be enlarged. Thereafter, the periphery of the existing tunnel is enlarged by utilizing the shaft as a base and the shaft is refilled with soil.
In another process, a cylindrical shield machine is driven along an ordinary tunnel and reaches a predetermined region to be enlarged. A cutter head of the shield machine is expanded with a special enlarging cutter member. Thereafter, the forward driving of the shield machine is carried out by operation of the cutter head with the special enlarging cutter member so that enlargement of the predetermined region of the tunnel can be performed.
However, the shaft process is defective in that extra work is required for constructing the shaft and refilling the shaft. In addition, this process requires a company to acquire the land for constructing the shaft. The shield machine process is defective in that if the increased diameter of the cutter head extends beyond a predetermined size, the cutter head cannot operate against the earth pressure acting thereon. In addition, this process requires work for attaching or detaching of the enlarging cutter member in order to excavate the enlarged tunnel or excavate the ordinary tunnel again. Further, the work required for constructing an ordinary tunnel and the work required for enlarging a portion of the ordinary tunnel cannot be carried out at the same time.